Comparative Anatomy
Hi, I want you to meet my friend Shoshanna. She's a Zebra Finch and she is very good at it. She's here to help me talk about comparative anatomy, which is the study of similarities and differences between the anatomies of animals. We study comparative anatomy because it helps us learn more about our evolution and our shared ancestry. Organisms have their evolutionary history written all over them, if you know what to look for. For instance, which of these two living organisms would you say I'm more closely related to? Shoshanna the finch? Or Gordon the plant? This isn't a quiz, but.... Sure, it is a quiz. It is the easiest quiz that you will ever take in your life.

(Intro)

Gordon is green and can make his own food with just sunlight, water and carbon dioxide. While Shoshanna can't make her own food, she has to move around to find stuff to eat, to escape predators, to find mates and poop on park benches. Just like me...except, not the pooping on park benches, I mean the moving around. So yeah, shocker, I am more closely related to a bird than to a plant. You get a gold star. So that one's obvious, but as the relationships between organisms get closer, the questions get a lot more interesting. So what IS an animal? I mean, I know you know what an animal is, but when you're looking at Shoshanna and me here, what clues you in to the fact that we are members of the kingdom Animalia?

Locomotion
(1:19) Two things: For starters, we're both moving. Locomotion is a really good sign that an organism is an animal, unless you're a sponge. Now, I know what you're thinking though: Protists, bacteria and archaea, they all move around using flagella and cilia. But they also only have one cell. It's the multi-cellular locomotion that's so peculiar and specific to animals.

Heterotrophy
(1:41) So animals move because of the second trait that we have in common: We're heterotrophs. We get our energy from eating other life forms. Locomotion also helps us avoid predation and seek out mates for reproduction. Now plants, they can mate by dispersing their seed to the wind or having an insect come by and fertilize them. But if land animals did that? Things would get like, really messy and gross. Some aquatic animals actually do just release their sex cells into their surroundings and cross their fingers and presumably close their mouths, and hope that somebody gets pregnant. So, since animals have to eat and move around, they've evolved anatomical forms that help them do those things. But obviously those forms aren't the same in all animals. For instance, in order to move, Shoshanna and I both have to be able to apply force to the ground or the air to propel ourselves. Here's me pushing off the ground with my feet. And now, here's Shoshanna applying force to the air with her wings, which keeps her afloat and moving. And if I had a shark in the studio with me, which thankfully I do not, so I'll just pretend to be a shark, my fins would apply force to the water, which would propel me forward.

Convergent Evolution
(2:40) Now, you have to be careful with this stuff, because even though similar body structures, like fins or wings or feet, can mean animals have a close common ancestor, it can also mean that the animals just evolved similar forms because that's the best structure for the job. When this happens, it's called convergent evolution. For example, a tuna, a penguin and a seal are all animals that spend all or a lot of their time in the water. One's a fish, one's a bird and one's a mammal, but all three of them have a suite of similar features, the most notable being a really sleek, fusiform body that can move through the water like nobody's business and fins for propelling those bodies. But of course those three animals have very different evolutionary origins. Each of these three marine animals have independently "converged" on similar body shapes because they live in the same environment and need to do the same sorts of things. So instances of convergent evolution can make linking physical structure of an animal to its evolutionary history a little bit tricky. Which is why, for a long time, nobody really put much stock in comparative anatomy as proof as evolution. That is, until Thomas Henry Huxley came along.


Bio-lography
(3:40) Thomas Henry Huxley was the Father of Comparative Anatomy and the Father of Modern Paleontology. And he invented the word "agnostic" to describe his spiritual views. And he was the first person to conclude that birds evolved from small carnivorous dinosaurs! I'm glad I'm sitting down for this. Plus, we have much respect for his facial hair. Huxley was born in England in 1825, and though he started out as a doctor, after serving as a ship surgeon on a voyage to Australia in his 20s, he took to studying marine invertebrates. During his voyage, he sent all of his papers back to England, and when he got home he found that he had become a kinda famous marine invertebrate expert and he was admitted into the Royal Society. Huxley made friends with some other hot-shot natural scientists, including Charles Darwin, and a few years later, when Darwin outlined his theory of evolution in On the Origin of Species, Huxley is reported to have said, "How extremely stupid not to have thought of that!" In fact, he became such a huge Darwin supporter, that everybody started calling him "Darwin's Bulldog" because he threatened to cut the fool who badmouthed evolution. This is a good one: Huxley said, when he was talking about On the Origin of Species, "Old ladies, of both sexes, consider it a decidedly dangerous book." You just got Huxslapped. With this new tool of the theory of evolution, and in part to help promote the theory of evolution, Huxley connected paleontology and biology together by looking for similarities in anatomy in the fossil record, where he found all kinds of interesting stuff. Like some really obvious similarities between prehistoric horse fossils and modern day horses, as well as between dinosaurs and birds, though nobody really bought his insights into the resemblance between birds and dinosaurs for another hundred years. And just in case you were still on the fence as to whether intelligence is heritable, Thomas Henry Huxley is the grandfather of Brave New World writer Aldous Huxley and of Sir Andrew Huxley, who won the Nobel Prize for Physiology or Medicine in 1963.

Now because all animals come from the same evolutionary origin, in addition to sharing some anatomical structures like Huxley studied, we're also built from the same rudimentary blueprint. Our cells work pretty much the same no matter what sort of animal we are. So while animals have different strategies for moving around and acquiring food, once the food is gotten, all animals break it down, turn it into useful energy and distribute nutrients, and eliminate waste in pretty similar ways, unless you're a sponge.

Tissues
(6:00) Each of those functions is performed by collections of cells that group together in the body to form tissues. There are 4 primary types of tissues in the human body: the epithelial tissue, the connective tissue, muscle tissue and nerve tissue.

Epithelial Tissue
(6:11) Epithelial tissue is formed by cells that bind very closely together. A layer of it covers every organ and lines the digestive tract to prevent crazy acids and poop and stuff from going where it's not supposed to go. Epithelial tissue can also produce the slippery fluid to let your organs slide over each other like the membrane that lines the inside of your ribs so that your inflating lungs don't build up friction as they expand.

Connective Tissue
(6:33) Most types of connective tissue are made up of fibrous strands of collagen protein, and it adds support and structure to your body and holds your parts together. Some examples of connective tissue include the inner layers of your skin, your tendons, ligaments, cartilage, and bone. But oddly enough, connective tissue isn't defined by it's ability to connect, but instead by the presence of an extra-cellular matrix, meaning that part of the tissue extends outside of the cell. And so, somewhat confusingly, blood and fat are also considered connective tissues.

Muscle Tissue
(7:01) Muscle tissue is made up mostly of two specialized proteins: actin and myosin, which can slide past one another and allow for movement. It also includes a bunch of other proteins, including that longest word-in-the-world one, titin.

Nerve Tissue
(7:14) And finally, there's nerve tissue, which generates and conducts electrical signals in the body. These electrical messages are managed by the nerve tissue in the brain and transmitted down the spinal cord to the rest of the body. Nerve tissue is made up of two types of cells: the neurons, which do the electrical work, and the glial cells, which insulate and support the neurons.

Organs
(7:32) These tissues are then organized into organs, which perform different functions in the body, and these organs work together in organ systems.

Organ Systems
(7:39) For instance, most animals have a digestive system made up of, like, a mouth and an esophagus and a stomach and intestines and an anus. And a lot of animals have a skeletal system made up of bones and tendons and ligaments and cartilage. We're going to be talking about each of these systems in a lot more detail in a few weeks. These organ systems, like many different kinds of anatomical structures, are shared by lots of different kinds of organisms, unless you're a sponge. Because about 1.6 billion years ago, an organism developed that had a digestive system and a muscular system, and suddenly that organism was in it to win it. That organism was the common ancestor for all animals today, and it's the reason me and Shoshanna here are gonna hang out at the animal family reunion.

Thank you for watching this episode of Crash Course Biology. If you want to check out any of the stuff we talked about, there's a table of contents over on the side. Or you can just re-watch the whole video and we'll love you extra much. If you have any questions for us, please leave them down in the comments below or you can get in touch with us on Facebook or Twitter. We'll see you next time.